Many types of input devices rely on measuring rotation or other movement of a knob, a dial or some other type of element which a human user can manipulate. A scroll wheel on a computer mouse is a familiar example of such a control. However, numerous other types of input controls also measure rotational and linear movement. For example, a trackball measures both the direction and amount that a user-movable ball is rotated about X and Y axes. A joystick measures the direction and amount by which a stick is pivoted about its base. A dial (e.g., a volume control dial) measures the amount by which a user has rotated that dial, as well as the direction of rotation. A mouse typically measures the direction and amount of motion along two orthogonal axes.
Movement-sensing controls convert the rotational or linear movement of a physical component into some type of signal. Existing controls perform this conversion (also known as encoding) in a variety of ways. Some controls rely on a potentiometer coupled to the moving element. Rotation or other movement of the element causes a corresponding increase or decrease in the electrical resistance of the potentiometer. That resistance is then used as a measure of the element's movement. Although potentiometers have advantages (e.g., low cost, ease of implementation), they can also generate noise and suffer from performance degradation as the potentiometer wears. Potentiometers may have limited accuracy, and may not be suitable where very precise measurement is required. Potentiometers are also not suitable for endlessly rotatable input controls such as mouse scroll wheels.
Another type of encoder uses a light source (e.g., an LED) and a light detector on opposite sides of an encoding wheel having alternating regions which block passage of light. The encoding wheel may be directly manipulated by the user or coupled to some other element which the user moves. As the encoding wheel turns, light from the LED is alternately received and then blocked from the light detector. The light detector output is then used to measure the amount by which the encoding wheel has rotated. These types of encoders can be used in an endlessly rotatable control, and have numerous other advantages. However, the precision of these encoders may also be limited. Moreover, these encoders usually require that two separate elements (the LED and the receptor) be aligned.
Optical imaging is another mechanism used for encoding motion. Typically, an LED or other light source illuminates a tracking surface (e.g., a desktop over which a mouse moves, a trackball outer surface, etc.). An imaging array is then used to capture an image of a portion of the tracking surface. Successive images are then correlated and used to determine direction and/or speed and/or amount of tracking surface movement. Although optical imaging represents a substantial advance in motion encoding, cost concerns may limit the resolution of imaging arrays used for motion encoding. There may also be limits upon the types of surfaces which a given imager may be able to track.
Laser Doppler velocimetry has been used in instrumentation applications such as measuring fluid flow, particle velocity and speeds of moving objects. One potential method of laser Doppler velocimetry makes use of the “self-mixing” effect. As is known in the art, the intensity of a laser's output will change if a portion of that laser's beam is reflected back into the laser's emitting cavity and mixes with the light being generated in the emitting cavity. The change in the laser's output intensity is a function of, e.g., the roundtrip delay between the time that light leaves the laser and the time that the light is reflected back into the emitting cavity. If the laser's beam is reflecting from a moving target back into its emitting cavity, the laser's power output will vary in a periodic manner. These power fluctuations, or “beats,” have a frequency which corresponds to the Doppler shift associated with movement of the reflecting target away from (or toward) the laser. Laser Doppler velocimetry can be very accurate and repeatable. However, the cost and complexity of laser Doppler velocimetry systems has remained high. For at least these reasons, such systems have not been used with computer or other types of low cost input devices.